Optical arrangements giving rise to a synthetic three-dimensional image or an image that changes its appearance at different angles have been used in many applications. Besides purely esthetical uses, such arrangements have been used e.g. as security labels on bank-notes or other valuable documents, identification documents etc. or as brand protection. The synthetic three-dimensional images have also been used for providing better geometrical understanding of complex shapes in e.g. two-dimensional information documents.
Synthetic image devices are based on the interaction between an array of focusing elements and micro image portions. In some prior art “synthetic images” are also referred to as “integral images” or “moiré images”, since the experienced image is composed of a number of parts interpreted together as an integral unit. The relative geometrical relation gives rise to different optical effects, from different levels of 3D to images that move or change its appearance e.g. dependent on the viewing angle. The focusing elements and the micro image portions are typically provided by printing on an essentially transparent polymer film.
One approach for manufacturing a synthetic image device is disclosed in the published International patent application WO 2011/102800 A1. Here a roll-to-roll process is achieved by the interaction of a matrix roll with a substrate. The matrix roll comprises recesses that are filled with a curable compound. The curable compound is transferred onto the substrate during a rolling contact between the matrix roll and the substrate. Typically, in order to facilitate the adhesion of the curable compound to the substrate, the surface of the substrate is pre-treated, e.g. provided with a surface layer. A good adherence between that surface layer and the curable compound in the matrix roll assists in removing the curable compound from the recesses when the rolling contact ends.
An image object in many applications of an optical arrangement that provides a synthetic integral can be coloured to obtain a black and white image, a greyscale image or a coloured image, or simply to provide proper optical properties. This colouring may typically be obtained by filling the recesses in the matrix roll with ink. This operation is a challenge, in particular in large-scale production using for example the above mentioned roll-to-roll arrangement, since the embossed cavities, ranging from μm-sized cavities to cm-sized cavities, should all be equally filled without leaving residual ink on intermediate surfaces.
When the ink is transferred to the substrate sheet, the ink may be spread outside the area defined by the recesses. This is particularly cumbersome if the substrate is provided with a surface layer for improving the adhesion properties. The contact between the substrate and the matrix roll is typically quite intense, at least at a micro level, and the contact force causes the surface layer to shear between the substrate and the matrix roll. This increases the risk for dragging ink out along from the recesses.
One way to mitigate such effects is to pre-cure the ink before it comes into contact with the surface layer of the substrate. The larger the printing speed is, the higher level of pre-curing is needed to counteract any shearing action of the surface layer. However, pre-curing of the ink within the recesses makes it in general more difficult to release the ink from the recesses and the risk that the ink becomes stuck in the matrix recesses thereby increases. These releasing problems also typically increase with increasing printing speed. If the ink is cured too much before the contact with the surface layer is established, any cross-linking between the surface layer and the ink will also be reduced, reducing the adhering effect of the surface layer.
Another problem is removal step for excess ink. First, the removal wiping causes wear, which shortens the life of the printing matrices. Secondly, in order to achieve a thorough wiping, the printing speed has typically to be limited. A too high printing speed will typically deteriorate the wiping efficiency and thereby the quality of the final product.
In other words, in prior art manufacturing approaches there might be ink-spreading problems and wear problems when they are used in high-speed manufacturing.